This invention relates to automobile headlight units containing incandescent lamp capsules, and more particularly, to incandescent lamps for such headlight units containing two filaments for high and low beam operation.
Heretofore, in the typical automobile headlight lamps manufactured in the United States for providing low and high beam operation, the lens has been designed principally for low beam operation, with the high beam, at best, a compromise. A typical low beam pattern generally has low intensity, wide spread, and very little light in the upper left hand area, as projected on a screen in front of the headlight. An ideal high beam pattern is of very high intensity with very little spread. The high beam is normally aimed straight ahead, along a line perpendicular to the lens face and through its center, as opposed to the low beam, which is usually directed somewhat downward and to the right when viewed from behind the headlight. Since both beams must come from the same combination of reflector and lens, ideal high and low beams cannot be readily achieved in the same lamp. Typically, U.S. headlight manufacturers have used a filament arrangement wherein the filaments are parallel to the road surface and orthogonal to the axis of the reflector; for example, see U.S. Pat. No. 3,898,451.
European headlight manufacturers, however, often use a filament arrangement wherein both filaments are mounted parallel to the reflector axis, and are axially displaced from each other. The high beam filament is usually at the focal point, with the low beam filament displaced axially forward of the high beam filament, i.e., away from the reflector. The low beam filament is usually also partially surrounded by a shield to reduce glare. For example, a typical European headlight lamp capsule, referred to as the "H4" type, is described in U.S. Pat. Nos. 3,646,385 and 3,646,386. This design tends to be somewhat inefficient on low beam due to the effects of the shield, and also due to the fact that the low beam filament is so far off focus.
Yet another filament arrangement is described in U.S. Pat. Nos. 3,493,806 and 3,569,693, wherein the low beam filament is axially disposed on the optical axis of the headlight, and the high beam filament is located behind the low beam filament (closer to the vertex of the reflector) and centrally disposed on but orthogonal to the optical axis. In U.S. Pat. No. 3,493,806, the filaments are disposed in a separate sealed lamp envelope with a screen means provided on the exterior surface of the envelope. U.S. Pat. No. 3,569,693 does not employ a sealed lamp capsule within the headlight but uses a shield between the low and high beam filaments.
U.S. Pat. No. 2,791,714 describes a dual filament arrangement in an airplane headlight for selectively projecting either a landing beam or a taxiing beam. This headlight employs a main high wattage filament which is axially disposed on focus along the optical axis of the headlight reflector. The headlight also includes a supplementary lower wattage filament in the form of a linear coil extending transversely of the reflector axis and disposed approximately in the focal plane of the reflecting surface. Further, the supplementary filament is disposed approximately symmetrical with respect to the vertical axial plane of the reflector and approximately parallel to the horizontal axial plane of the reflector. The supplementary filament is operated in parallel with and positioned horizontally and above the main filament to provide the landing beam. In operation, the lamp is connected in an operating circuit which is adapted to selectively connect either the supplementary filament alone or the two filaments in parallel across an electric power supply. When the line voltage is impressed across the horizontally disposed upper or supplementary filament alone, a relatively wide flood or taxiing beam of the required lateral spread is produced. When the line voltage is impressed across the main filament and the supplementary filament in parallel, a landing beam is produced having a generally circular shaped central hotspot portion with a slightly depressed wide spread portion of lower candle power to provide forground illumination.
In recent years, for styling and other considerations, rectangular headlights have come into vogue. Prior to this, domestic headlamps used reflectors of essentially parabolic cross-section, circular in shape, thus forming a paraboloid. Rectangular reflectors are also essentially paraboloidal but have a portion of the top and bottom of the reflector truncated, as shown in FIGS. 2 and 3. Both the round and rectangular domestic headlamps have typically used the parallel filament arrangement discussed hereinbefore with reference to U.S. Pat. No. 3,898,451.
Examination of the intensity distribution of a typical single-coil filament reveals that the radiation is maximum in a direction perpendicular to the axis. If this filament is placed in a round reflector with its length aligned perpendicularly to the axis of the reflector, maximum flux is emitted in those areas of the reflector which lie perpendicular to the length of the filament. Looking into the reflector from the front, one would see virtual images of the filament in various areas of the reflector, corresponding to the orientation of the filament. The projected image of the filament on a suitable distant screen produces the well known "bow-tie" pattern, the "knot" of which represents radiation from the central on-focus portion of the filament, and the "wings" representing the radiation from the off-focus ends of the filament. Placing this filament in a rectangular reflector in the usual horizontal orientation is essentially the same as truncating the round reflector in the previous example to a rectangular shape. Thus, the slight radiation from the "cold" ends of the filament is directed at those areas of the reflector with the highest flux collection efficiency. This situation could be rectified somewhat by rotating the filament 90.degree. into a vertical orientation, but the spread light, which would be spread vertically, would significantly increase the beam intensity well above the horizon. This can be very objectionable because it has the potential of producing back scattered light under certain driving conditions, such as rain, fog or snow.
However, aligning the filament coaxially with the reflector axis distributes the filament flux symmetrically about the reflector, with the respective images of the filament radially disposed in the upper portion of the reflector. With this distribution, those areas of the reflector which are not truncated are put to better use. The projection of these images on a screen would be as a target centered below and to the right of the center of the screen coaxial with the optic axis of the headlight. This circular pattern has its highest intensity at the center and decreasing intensity radially outward from the center.
Since the high beam is used to see far ahead, only the light coming out of the headlight in a small cone is of much use. With this in mind, an experiment was performed to compare the quantity of light delivered into a cone with a total included angle of about 14.degree. by similar filaments with axial and horizontal orientations in a typical rectangular reflector. The result was that in the 14.degree. cone, the axially oriented filament delivered approximately 10% more light. Thus, a 10% efficiency gain is realized. Also due to the geometry of the axial filament orientation, the ends of the filament, which are off-focus, have their magnified projected images superimposed rather than diametrically opposed as in the case of the transverse filament. This effectively decreases the main beam spread, thereby increasing the maximum beam intensity attainable from a similar filament mounted in the usual transverse fashion. Thus, with the use of the axially mounted high beam filament, as described hereinafter in accordance with the present invention, both the maximum intensity and efficiency increase. These changes are very desireable since the energy of the high beam can better be concentrated down the road, improving the seeing distance while utilizing no additional energy.